Quest To Demystify Water: Ideal Solution Behaviors Are Obtained by Adhering to the Equilibrium Mass Action Law

The current belief that water solutions are not "ideal" is due to the assumption that the mass action law is not obeyed and activity coefficients are used to avoid violating an immutable law. The law is not violated when expressed by including waters of hydration as part of the reactants and as products in the formation of ion pairs by electrolytes. Mole fractions calculated by adhering to the law and recognizing effects of incomplete dissociation and amounts of water interacting strongly with solutes obey Raoult's law to high concentrations. Another belief is that, for a solution to be "ideal", there must be no volume or heat change on mixing the components. We demonstrate "ideality" despite volume changes and nonzero enthalpies of solution. Results with a variety of electrolytes correlate with residence times of waters near ions, viscosities, entropies of hydration, enthalpies of solution and dilution, ion ionization energies, Hofmeister series, and so forth. Older reports of "negative hydration", that had been considered unreliable, are now understood; the effect is due to formation of mobile, or "fast", water by strong hydrophobic associations. Our results are supported by findings from techniques perfected relatively recently: dielectric relaxation, mid-IR pump-probe spectroscopy, 2D-IR, atomic force microscopy, and so forth.